1. Field of the Invention
The present invention relates to a threshold element used in a logical operation circuit. More specifically, the present invention relates to a threshold element using a MIS (metal insulator semiconductor) transistor.
2. Description of the Related Art
As conventional threshold elements, threshold elements based on output-wired CMOS (complementary metal-oxide semiconductor) inverters have been known. FIG. 8 shows a configuration of a conventional threshold element using an output-wired CMOS inverter. In this element, each of inputs X.sub.1, X.sub.2, . . . X.sub.n has a corresponding inverter 1, 2, . . . n. Outputs from these inverters are all wired together, creating an output voltage Vout. The output voltage Vout is input to a comparing inverter 10. A threshold value of the comparing inverter (judging inverter) 10 is set appropriately so that a majority result can be converted into binary data properly. In this threshold element, the output voltage Vout reflecting a sum of the outputs from the inverters 1 through n corresponding to the inputs X.sub.1 through X.sub.n is compared with a threshold value by the comparing inverter. If the output voltage Vout is larger than the threshold value, "1" is output as a final result y. Otherwise, "0" is output. The threshold element can be used for various kinds of logical operations, by applying an appropriate weight to each of the inputs X.sub.1 through X.sub.n and setting a suitable threshold value to the comparing inverter 10.
In the above-described conventional technique, at least 2 transistors are required for each input, which leads to a problem that a large area on a substrate is occupied by the threshold circuit. Furthermore, weighting each inverter for each input as well as designing and setting the proper threshold value for the judging inverter are complicated.
An object of the present invention is therefore to provide a threshold element enabling a logical operation with fewer transistors, easy element weighting, and easy design and setting of the threshold value.